1. Field of the Invention
This invention relates generally to the field of air valves and more particularly to the use of such valves in connection with compressed air powered acoustic pulse generators.
2. Description of the Prior Art
U.S. Pat. No. 4,284,165 issued Aug. 18, 1981 to Airhart, et al. describes generally an apparatus for generating an acoustic pulse wherein an impact piston upwardly latched in a firing position is powered downwardly by compressed air or gas to strike a target plate positioned against the earth. The compressed air is admitted into the piston housing through ports penetrating the housing side wall near its top. The upper portion of the piston is tapered and in its upward or firing position, as supported by means of an electrically controlled latch or latches, is designed to press against static seals on the correspondingly tapered portion of the housing bore above and below the side wall ports. Since the tapered wall of the piston is continuously exposed to air pressure, a positive breakaway force is exerted downwardly at all times upon the piston. The tapered piston wall also insures that air pressure is released within the housing bore to drive the piston downwardly the instant the piston disengages the side wall seals.
Although the above-described positive breakaway force has the advantage of ensuring a positive piston release, it necessarily applies a load tending to impede the smooth, repeatable operation of the latch, whether the support is from the bottom or the top. This force is in addition to that produced by the dead weight of the piston mass. It has been found that as the applied air pressure is increased, the time required for latch disengagement begins to become erratic. This in turn means that even if the applied air pressure is maintained constant, the elapsed time between the electrical signal and target impact will vary. In seismic work, it is frequently desirable to fire a group of seismic sources simultaneously in order to increase the strength of the acoustic pulse to be injected into the earth. In such cases, a few milliseconds variation in the instant of target impact of multiple sources will seriously degrade informational content and proper interpretation of the seismic reflections. The greater the applied air pressure, the greater these variations tend to become.
It is therefore a general object of this invention to provide a compressed air powered acoustic generator with improved means for controlling the initiation of a firing operation.
It is a further object of this invention to provide a compressed air powered impact-type acoustic pulse generator of the type described with highly reproducible timing characteristics over a range of applied air pressure.
Other objects and advantages of this invention will become apparent from a consideration of the detailed description to follow taken in conjunction with the accompanying drawings and the appended claims.